1. Technical Field
The embodiments herein generally relate to radio frequency (RF) technologies, and, more particularly, to downconverting a wanted RF signal in the presence of undesirable RF signals in a RF network using a wideband resistive input mixer.
2. Description of the Related Art
Orthogonal frequency-division multiplexing (OFDM) technology relates to digital modulation for minimizing interference by multiple-path or fading channels near each other in frequency. For a wireless communication device, a radio frequency (RF) signal is typically received, filtered, and frequency converted. The spectrum input to RF devices typically includes a large number of undesired signals (blockers) in addition to the desired band of interest. Such interferences can be very large, possibly causing intermodulation distortion, desensitization, cross-band modulation, and oscillator pulling, among other undesirable effects.
Most typical RF receivers require a band-limiting filter at their input to eliminate or reduce such interferences. These filters typically require very high selectivity (i.e., a very narrow passband relative to the filter center frequency). In certain wide-band applications, these filters must move to track the desired channel. There are generally two conventional approaches to RF filtering. In applications where tracking is not required, an off-chip resonator such as a surface acoustic wave (SAW) filter is employed.
The benefit of these filters is excellent selectivity and accurate passband location. However, the disadvantages are twofold. First, these filters generally have approximately 2 dB loss in their passband. This translates to an additional 2 dB of noise figure (NF) and thus directly affects the minimum possible sensitivity of the system. Second, these filters invariably add significant cost to the bill of material (BOM) and generally increase the circuit board area.
For tracking applications, a tuning element such as a p-type intrinsic, n-type diode (PIN diode) is used to tune the resonance of a tank or similar resonant circuit. While this approach manages to provide a tunable filtering, it generally suffers from poor stopband attenuation and less passband frequency accuracy than SAW filters. Furthermore, these filters are off-chip, and again impact BOM costs. Finally, such “tracking” filters must be very carefully tuned or they may unintentionally attenuate the desired signal. As a result, factory calibration/tuning is generally required, leading to more cost and complexity of implementation.
Equally important to RF selectivity and filtering is the receiver linearity. Greater linearity offers improved resistance to intermodulation distortion and compression, and allows the demodulation of small signals in the presence of large blockers. Typical designs have widely varying linearity, with cascaded IIP3 ranging from −20 dBm up to +5 dBm. Generally, higher IIP3 designs use more power, which is problematic in portable or battery powered applications. Improving the system linearity will directly improve the ability to receive in the presence of blockers and/or the power consumption of the system.
Finally, in wideband designs achieving a good broadband resistive match typically comes at the expense of noise. Commonly used techniques have >4 dB noise figure, which directly degrades the system sensitivity. As a result providing low-noise or “noise-cancelled” resistive input impedance is of great utility.